The last window of same-sex marriage in the state occurred between June 2008, when a California Supreme Court ruling allowing gay weddings under the state Constitution went into effect, and Nov. 4, 2008, when voters approved Proposition 8.

About 18,000 same-sex couples married during that time, and the state high court left those marriages in place at the same time that it upheld voters' right to enact Proposition 8 in a decision in May 2009.

The two couples who challenged California's same-sex marriage ban in federal court last year, Kris Perry and Sandy Stier of Berkeley and Paul Katami and Jeff Zarrillo of Burbank, claim the measure violates their right to equal treatment and their fundamental right to marry.

Another, narrower argument by the plaintiffs that would apply only to California is that California has a uniquely unequal situation because some same-sex couples are married while others cannot marry.

The initiative's sponsors contend it is constitutional because it is a reasonable way of preserving the traditional definition of marriage and promoting the welfare of children.

There will be no court hearing when Walker issues the ruling. The decision will be posted on the court's website at www.cand.uscourts.gov, and a limited number of paper copies will be available at the court clerk's offices in San Francisco, Oakland and San Jose.